A semiquantitative theory for the 2fp radiation observed upstream from the Earth's bow shock is presented: the radiation is produced by the process L+L→T+S, proceeding as two sequential three-wave steps L→L'+S and L+L'→T, in the foreshock where nonthermal L and S waves are observed. (Here L, S, and T denote Langmuir, ion acoustic, and transverse electromagnetic waves, respectively.) This theory is consistent with all the available wave data, including the characteristics and levels of a class of low-frequency waves identified as S wave products of the process L→L'+S, and the brightness temperature and bandwidth of the 2fp radiation. Indeed the theory could account for higher 2fp brightness temperatures if required. Predictions of the theory suitable for observational testing include (1) the existence of two 2fp sources, one to each wing of the foreshock, (2) the spatial location of the source regions, and (3) the characteristics and levels of the product L' and S waves in the source regions. The radiation should (4) have intrinsic bandwidths of the order of 1 kHz or less, (5) be less than 0.1% circularly polarized, and (6) have a limiting brightness temperature equal to the effective temperature TL of the L wave producing the radiation. ¿ American Geophysical Union 1988